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The first series of TV3's science fiction drama Humans was so good you almost hoped there wouldn't be another.

It's the second series where high-concept shows like this so often come undone. The writers run out of ideas, the storylines get weaker and more convoluted and eventually the whole thing collapses in on itself. You keep digging through the confusing rubble for a few episodes, hoping to find what it was you once loved about the show, but by then it's always too late.

Some shows are just meant to run for one perfect series. The initial eight episodes of Humans offered a neatly paced and deceptively complex narrative, which was satisfyingly resolved in the finale. It felt like a minor miracle. But it also left the door wide open for more - in fact the door was taken off its hinges and upcycled it into a trestle table, such was the scope of possibility.

Set in a parallel universe where lifelike humanoid robots - "synths" - exist as domestic and industrial workers, the series introduced five fugitive synths who had been programmed by their mysterious maker to feel human emotion, and who carried inside them the code required to give all synths consciousness.

The series was anchored by the very real, very human Hawkins family, who unknowingly bought one of the five special synths, Mia, after she had been kidnapped and reprogrammed. With the Hawkins asking all the questions and experiencing all the dilemmas of life with synths on our behalf, the show's science fiction element always felt firmly tied to reality.

A big part of the intrigue lay in the mysteries of the past, the gradual unravelling of how these synths came to be and how all the characters related to one another. Now that we know the Edward Scissorhands-style back-story, the big mystery of series two lies in the future: what happens when all other synths become sentient?

A year or so further on from the last episode, Niska (the blonde synth who was working in a brothel before she went rogue and killed a client, sparking the intense synth-hunt at the core of the first series) is living as a human in Berlin. She is in possession of a very important hard drive - the one with the code that could wake up every synth in the world.

Where the scope of the first series was deliberately small - one family, one group of synths - series two goes big picture. The synth awakening is global: first a worker down a mine in Bolivia gains consciousness, then another working in a factory in Nottingham.

Those ones find their way to the tremendously cagey synth guru Leo who, with series one heroes Max and Mia, is hiding out in an abandoned farmhouse in the south of England - the three survival experts help the newcomers avoid capture and adjust to the sentient life.

Others fall into seemingly more sinister hands - in San Francisco, a tech giant tries to lure a leading synth academic over to the dark side to help reverse-engineer a conscious synth in his possession.

The Hawkins family still have a role to play, too. The synth skeptics from series one now find themselves important synth rights advocates - even if the dad has just been made redundant, his job restructured into a "non-human role".

The way the episode so easily draws us back in with each new strand of the story suggests that although its scope is much greater this time, the show's best qualities remain - it is still a uniquely fine-tuned blend of science fiction and family drama, full of characters written with wry humour and real emotion.

If anything, the second series of Humans could turn out even better than the first.